The debate over the best and most practical joinery method never dies. Which joints are strongest? Is it always necessary to use the strongest joint possible, no matter how long it takes? A biscuit joiner can certainly speed things up, but a biscuit joint doesn’t come close to matching the strength of a traditional mortise and tenon joint. When is taking the fast and easy route acceptable, and who gets to decide? One Woodworker’s Journal eZine reader was curious, and got an especially thoughtful response from a guy who oughta’ know.
"The larger question asks about guidelines for making decisions about the strength of joints.
"What we do is make our judgment based on empirical values; we rely on experience and observation. The outcome is that we usually make the joint overly strong. We have no idea by how much it's overbuilt, but as long as it doesn't break during its useful life, that's OK. Mortise and tenon type joints are the ones we use to join parts together to make structures that have to withstand stress – chairs, beds, tables and the like. Maximizing joint strength seems to me to be a good idea – knowing more about it was what caused me to put forward a proposal for a dissertation whilst studying for my Wood Science & Technology degree. It's now more years ago than I care to think about, but loosely said, it was a comparison of the strength of mortise and tenon joints and dowel joints.
"Well, the proposal got nixed because it was determined to be "insufficiently academic." Instead I did research on some microscopic comparison between two rare species of softwood – a subject of interest to me and two other people in the world. A couple of years later, FIRA (The Furniture Industry Research Association), a British organization, decided to fund research into what amounted to the same study I had proposed for my first dissertation.
"The research was funded for one year – no report. Then it was funded for a second year – no report. Then a third year; the result was "there are altogether too many variables for the data to be of any practical value to a woodworker." Here is a sampling of the variables:
• How rough or smooth are the walls of the tenon, the mortise and the dowel hole?
• How round is the hole?
• How round is the dowel?
• How tight or loose are the interfaces of the parts?
• What glue was used?
• How thoroughly were the parts wetted with the glue?
• What species of wood was used?
• What was its growth rate?
• Were the joint parts tangential or radial tissue?
• What was the moisture content of the wood?
"And so it remains.
"In a mortise and tenon type joint, there are two things to take into account with regards to strength: the mechanical strength and the glue line strength. To get a gauge of the mechanical strength, put the pieces together dry and test the sort of stress it would take to cause them to come apart. In other words, how much does the joint rely on the glue in order to stay together?
"To illustrate the point, I'll briefly describe four of the options you generally have with this sort of joint situation: a traditional M&T, a loose tenon, a dowel joint and a biscuit joint.
"Of the four, the traditional M&T is the only two-part joint. The others all bring into play a third element. Since the sizes and sections of the parts vary, each mortise and tenon is designed to be as strength-balanced as possible. The tenon part is no stronger than the mortise part. It's also designed to have the maximum glue area possible.
"A loose tenon has much the same strength as a mortise and tenon. What you begin to realize now is that form doesn't follow function – form follows economy. You only need one machine to make a loose tenon: a slot mortiser. The question is: can you afford one?
"In spite of all the theoretical disadvantages and failings of a dowel joint, it mostly works.
"The biscuit joint has quite a bit less mechanical strength than the other three and its glue area isn't great. But it scores big on the accuracy of its glue line. The walls of the joint are smooth, the plate swells to form a tight interface and, in many cases, it's possible to double up the biscuits so the gluing area is doubled.
"At the end of the day, whatever you decide, you must ask: Is the joint sufficiently strong to outlast the useful life of the piece?"
- Ian Kirby
From the Woodworker’s Journal eZine archives
If you favor the strength of the mortise and tenon joint, and like the blend of speed and strength that the loose tenon method offers, you have a couple of options.
The recently introduced Festool Domino Joinery System is the one of the most talked woodworking tool to come down the pike in years, and certainly the slickest way to cut a mortise that we’ve ever seen. The Domino (coming soon to Rockler) strongly emulates a biscuit joiner, both looks and operation, blending the speed of biscuit joinery with the mortise and tenon’s undeniably superior strength. But it’s not for everyone. It’s a high end system designed for woodworkers who are seriously involved in their craft. But while it isn’t exactly cheap, all reports thus far seem to say that it’s worth every penny.
The BeadLock system is another great solution. It’s a little more affordable, if not quite as speedy. If you already own a drill, all you need to get started is the simple, straightforward Beadlock jig, and either a few of the pre-made specially shaped loose tenons, or a Beadlock tenon cutting router bit.
Of course, there are quite a few other ways to accomplish the same thing. Many people cut mortises with a router. That can be a fairly efficient method, and most woodworkers already have a good start on the necessary equipment – a plunge router and a few straight router bits. All that's missing is a mortising jig, which you can either build or buy. Or if time and money are both major concerns, here’s a handy gadget that’ll get you started for around $20: The Mortise Centering Router Base fits most plunge routers and automatically positions a straight router bit for a perfectly centered mortise every time.